Method and apparatus for trending a physiological cardiac parameter

ABSTRACT

The present invention relates to an implantable cardioverter-defibrillator or pacemaker whose standard circuitry is used to trend a physiological cardiac parameter using intra-cardiac impedance measurements. The trend information may be used to predict the onset of a sudden cardiac death (SCD) event. By being able to predict the onset of an SCD event, patients and their physicians may be forewarned of a life-threatening event allowing them to respond accordingly. The trend information may also be used to predict the efficacy of cardiac-related medications, monitor progress of congestive heart failure, detect the occurrence of myocardial infarction, or simply track changes in sympathetic tone.

FIELD OF INVENTION

[0001] The present system relates generally to implantablecardioverter-defibrillators and pacemakers and particularly, but not byway of limitation, to such systems being used to trend a physiologicalparameter using intra-cardiac impedance measurements.

BACKGROUND OF THE INVENTION

[0002] The heart is generally divided into four chambers, the left andright ventricles and the left and right atria. Blood passes from theright atrium into the right ventricle via the tricuspid valve. Theatrial chambers and the ventricular chambers undergo a cardiac cycleconsisting of one complete sequence of contraction and relaxation of thechambers of the heart. The term systole describes the contraction phaseof the cardiac cycle during which the ventricular muscle cells contractto pump blood through the circulatory system. The term diastoledescribes the relaxation phase during which the ventricular muscle cellsrelax, causing blood from the atrial chamber to fill the ventricularchamber. After completion of the period of diastolic filling, thesystolic phase of a new cardiac cycle is initiated.

[0003] Through the cardiac cycle, the heart is able to pump bloodthroughout the circulatory system. Effective pumping of the heartdepends upon five basic requirements. First, the contractions of cardiacmuscle must occur at regular intervals and be synchronized. Second, thevalves separating the chambers of the heart must fully open as bloodpasses through the chambers. Third, the valves must not leak. Fourth,the contraction of the cardiac muscle must be forceful. Fifth, theventricles must fill adequately during diastole.

[0004] When functioning properly, the human heart maintains its ownintrinsic rhythm based on physiologically-generated electrical impulses.However, when contractions of the heart are not occurring at regularintervals, or are unsynchronized, the heart is said to be arrhythmic.During an arrhythmia, the heart's ability to effectively and efficientlypump blood is compromised. Many different types of arrhythmias have beenidentified. Arrhythmias can occur in either the atria or the ventricles.Arrhythmias may be the result of such conditions as myocardialinfarction, cardiomyopathy or carditis.

[0005] Ventricular fibrillation is an arrhythmia that occurs in theventricles of the heart. In ventricular fibrillation, various areas ofthe ventricle contract asynchronously. During ventricular fibrillationthe heart fails to pump blood. If not corrected, the failure to pumpblood and thereby maintain the circulation can have fatal consequences.

[0006] Ventricular tachycardia is an arrhythmia that occurs in theventricular chambers of the heart. Ventricular tachycardias are typifiedby ventricular rates between 120-250 beats per minute and are caused byelectrical or mechanical disturbances within the ventricles of theheart. During ventricular tachycardia, the diastolic filling time isreduced and the ventricular contractions are less synchronized andtherefore less effective than normal. If not treated quickly, aventricular tachycardia could develop into a life-threateningventricular fibrillation.

[0007] Supraventricular tachycardias occur in the atria. Examples ofthese include atrial tachycardias, atrial flutter and atrialfibrillation. During certain supraventricular tachycardias, aberrantcardiac signals from the atria drive the ventricles at a very rapidrate.

[0008] Sudden cardiac death (SCD) may be a consequence of cardiac rhythmabnormalities occurring in the ventricles or the atria such asventricular fibrillation, ventricular tachycardia or one of thesupraventricular tachycardias. Sudden cardiac death fatally afflictsabout 300,000 Americans each year.

[0009] Patients with chronic heart disease can receive implantablecardiac devices such as pacemakers, implantablecardioverter-defibrillators and HF cardiac resynchronization therapydevices. Implantable cardioverter-defibrillators (ICDs) are used asconventional treatment for patients whose arrhythmic conditions cannotbe controlled by medication. These devices provide large shocks to theheart in an attempt to revive a patient from a cardiac rhythmabnormality that may result in an SCD occurrence. At the present thereare no firm predictors for SCD within these devices.

SUMMARY OF THE INVENTION

[0010] This document discusses an implantable cardioverter-defibrillatoror pacemaker whose standard circuitry is used to trend a physiologicalcardiac parameter using intra-cardiac impenance measurements. The trendinformation may be used to predict the onset of an SCD event. By beingable to predict the onset of an SCD event, patients and their physiciansmay be forewarned of a life-threatening event allowing them to respondaccordingly. The trend information may also be used to predict theefficacy of cardiac-related medications, monitor progress of congestiveheart failure, detect the occurrence of myocardial infarction, or simplytrack changes in sympathetic tone.

[0011] In one embodiment of the present invention, a method ofpredicting sudden cardiac death includes the steps of determiningintra-cardiac impedance, deriving a physiologic cardiac parameter fromthe determined impedance, trending the parameter over spaced timeintervals, and predicting the onset of a sudden cardiac death episode.

[0012] In another embodiment, a system for predicting sudden cardiacdeath episode includes a device that measures intra-cardiac impedance, aderivation module that derives a physiological cardiac parameter fromthe measured impedance, and a module that trends the derived parameterover spaced time intervals to create trend data. The system may alsoinclude an analyzing module that analyzes the trend data to predict asudden cardiac death episode.

[0013] In a further embodiment, a method of trending a cardiac parameterincludes the steps of measuring an intra-cardiac impedance, determininga physiologic parameter using the intra-cardiac impedance, and trendingthe cardiac parameter over time.

[0014] In a yet further embodiment, a device for trending aphysiological cardiac parameter includes an impedance module thatmeasures intra-cardiac impedance, a parameter module that calculatescardiac parameter values using the measured impedance, and a trendingmodule that generates trend data using cardiac parameter values.

[0015] These and various other features, as well as advantages, whichcharacterize the present invention, will be apparent from a reading ofthe following detailed description and a review of the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] In the drawings, which are not necessarily drawn to scale, likenumerals describe substantially similar components throughout theseveral views. Like numerals having different letter suffixes representdifferent instances of substantially similar components. The drawingsillustrate generally, by way of example, but not by way of limitation,various embodiments discussed in the present document.

[0017]FIG. 1 is a schematic/block diagram illustrating generally, amongother things, one embodiment of portions of an impedance sensor fortrending a physiological cardiac parameter and an environment in whichit is used.

[0018]FIG. 2 is a schematic/block diagram illustrating generally, amongother things, one embodiment of portions of an impedance sensor fortrending a physiological cardiac parameter.

[0019]FIG. 3 is a schematic/block diagram illustrating generally, amongother things, one embodiment of further portions of the measuring moduleof the impedance sensor of FIG. 2.

[0020]FIG. 4 is a schematic/block diagram illustrating generally, amongother things, an embodiment of further portions of the parameter moduleof the impedance sensor of FIG. 2.

[0021]FIG. 5 is a schematic/block diagram illustrating generally, amongother things, an embodiment of further portions of the trending moduleof the impedance sensor of FIG. 2.

[0022]FIG. 6 is a schematic/block diagram illustrating generally, amongother things, an embodiment of further portions of the analyzing moduleof the impedance sensor of FIG. 2.

[0023]FIG. 7 is a schematic/block diagram illustrating generally, amongother things, another embodiment of portions of the an impedance sensor.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0024] In the following detailed description, reference is made to theaccompanying drawings which form a part hereof, and in which is shown byway of illustration specific embodiments or examples. These embodimentsmay be combined, other embodiments may be utilized, and structural,logical and electrical changes may be made without departing from thespirit and scope of the present invention. The following detaileddescription is therefore, not to be taken in a limiting sense, and thescope of the present invention is defined by the appended claims andtheir equivalents.

[0025] The present system and methods are described with respect toimplantable cardiac rhythm management (CRM) devices, such as pacemakers,cardioverter defibrillators (ICDs), pacer/defibrillators, andmulti-chamber and/or multi-site (in a single chamber or multiplechambers) cardiac resynchronization therapy (CRT) devices that utilizestandard pacing and defibrillating leads. The software directingoperation of such devices may be modified in a way to utilizeintra-cardiac impedance measurements collected by the device to generatea physiological cardiac parameter. The device may also be programmed totrend the generated parameter over time. The trend information mayrepresent changes in sympathetic activity of cardiac tissue and therebybe used to track certain physiologic indicators such as, for example,the prediction of a sudden cardiac death (SCD) event, the efficacy ofcardiac-related medications being taken by the patient, the detection ofa myocardial infarction, or the progress of congestive heart failure ina patient. For example, one trend may show a slow decrease in overallsympathetic activity over time, while another trend may show a sharpdrop in sympathetic activity that is sustained for a given period oftime, while yet another trend may show spikes of sympathetic activity atcertain times during each day that may be related to how the heart isreacting during specific activities. Because certain trends may indicatea specific physiological indicator (as listed above), the system of thepresent invention may be configured to identify the occurrence ofcertain physiological indicators from trend information. Suchphysiologic parameters may be referred to as “predeterminedphysiological indicators” to the extent that the system may beconfigured to identify and track one or more specific indicators basedon the trend information.

[0026] Sympathetic activity refers to the level of activation of theautonomic nervous system, specifically the sympathetic nerves thatregulate cardiac muscle contraction. Increased sympathetic activity (ortone) is an important contributor to the generation of spontaneouslife-threatening arrhythmias and SCD. Changes in sympathetic activityduring specific patient activities (such as exercise or sleep) over timemay provide important information for the patient and their physician.

[0027] There are several physiological cardiac parameters that may begenerated from intra-cardiac impedance measurements that provide insightinto sympathetic activity by inferring their effects on cardiaccontractility. Three exemplary parameters are stroke volume, ejectionfraction, and pre-ejection period (PEP). “Stroke volume” refers to thevolume of blood pumped from a ventricle of the heart in one beat.“Ejection fraction” refers to the ratio of the volume of blood the heartempties during systole to the volume of blood in the heart at the end ofdiastole expressed as a percentage. “Pre-ejection period” measures thelatency between the onset of electromechanical systole, and the onset ofleft-ventricular ejection.

[0028] In one example, it is known that PEP shortens when sympatheticactivity is increased. This shortened parameter may be measured viaintra-cardiac impedance. Therefore, should a patient experience amyocardial infarction (MI), or have already experienced a MI, electricalremodeling will occur in the heart. This remodeling may manifest itselfas an increased average sympathetic activity (detected by the shortedPEP values over some time interval), and eventually a life-threateningarrhythmia and possibly even sudden cardiac death.

[0029] The following is a detailed description of various systems andmethods of generating and trending physiological cardiac parametersbased on intra-cardiac impedance that are used to track certainphysiological indicators. FIG. 1 is a schematic/block diagramillustrating generally one embodiment of portions of a system 100 of thepresent invention and an environment in which it is used. In thisembodiment, system 100 includes, among other things, an CRM device 105,which is coupled by leads 110, 112, 137 to heart 114. Heart 114 includesfour chambers: right atrium 116, right ventricle 118, left atrium 120and left ventricle 122. Heart 114 also includes a coronary sinus 124, avessel that extends from right atrium 116 toward the left ventricularfree wall, and which, for the purpose of this document, is considered toinclude the great cardiac vein and/or tributary vessels.

[0030] Lead 110 may include an electrode associated with right atrium116, such as a tip electrode 126 and/or ring electrode 128. Theelectrode is “associated” with the particular heart chamber by insertingit into that heart chamber, by inserting it into a portion of theheart's vasculature that is close to that heart chamber, by epicardiallyplacing the electrode outside that heart chamber, or by any othertechniques of configuring and situating an electrode for sensing signalsand/or providing therapy with respect to the heart chamber.

[0031] Lead 112, which is introduced into coronary sinus 124 and/or thegreat cardiac vein or one of its tributaries, includes one or aplurality of electrodes associated with left ventricle 122, such as tipelectrode 130 and/or ring electrode 132. Lead 137 includes one or aplurality of electrodes associated with the right ventricle, such as tipelectrode 138 and/or ring electrode 140.

[0032] Device 105 may also include other electrodes, such as housingelectrode 134 and/or header electrode 136, which are useful for, amongother things, unipolar sensing of heart signals or unipolar delivery ofcontraction-evoking stimulations in conjunction with one or more of theelectrodes 126, 128, 130, 132, 138, 140 associated with heart 115.Electrodes 134 and 136 may be referred to in the art as “can”electrodes, such that electrodes 126, 128, 130, 132, 138, 140 positionedin the heart may be compared to or communicate with the “can”electrodes. Alternatively, bipolar sensing and/or therapy may be usedbetween electrodes 126 and 128, between electrodes 130 and 132, betweenelectrodes 138 and 140, or between any one of the electrodes 126, 128,130, 132, 138, 140 and another closely situated electrode. In practice,any combination of unipolar and bipolar electrodes positioned within theheart may be used, in addition to combining the electrodes positionedwithin the heart with “can” electrodes to obtain the necessary impedancemeasures.

[0033] Device 105 may include several features that may be representedby modules, process steps and components as hereinafter described. Forexample, device 105 may include a measuring module 142 that is coupledto one or more of the electrodes 126-136 for sensing electricaldepolarizations and intra-cardiac impedance corresponding with heartchamber contractions. Device 105 may also include a parameter module144, a trending module 146, an analyzing module 148, and other modulesor features relevant to tracking intra-cardiac impedance and trendingderived physiologic parameters over time. For example, device 105 mayinclude a transceiver 150 for communication between device 105 and anoutside source such as, for example, an external programmer 152, anexternal storage device 154, or an external analyzing module 156.

[0034] Referring now to FIG. 2, one embodiment of an example system ordevice 200 for trending a physiological cardiac parameter is provided.System 200 may include a measuring module 210, a parameter module 230, atrending module 250, and in some cases may further include an analyzingmodule 270. Modules 210, 230, 250 and 270 are further described hereinwith reference to FIGS. 3-6. In essence, the measuring module 210 iscapable of measuring intra-cardiac impedance values in a patient, theparameter module 230 is capable of calculating or otherwise deriving aphysiologic cardiac parameter using the measured impedance values, thetrending module 250 is capable of generating trend data using thederived parameter values, and the analyzing module 270 is capable ofanalyzing trend data to track predetermined physiological indicators. Insome embodiments, analyzing module 270 is part of a device includingmeasuring, trending and parameter modules, such as the device 105 shownin FIG. 1. In other embodiments, analyzing module 270 may be an externalanalyzing module, such as module 156 illustrated in FIG. 1, thatanalyzes trend data at a separate location from the device in which themeasuring, parameter and trending modules are located. Also, in otherembodiments, system 200 may include other modules or components such asa transceiver 150, a controller (not shown), a signal generator (notshown), etc. if such components or modules are not integrated into themeasuring, parameter, trending and analyzing modules.

[0035]FIG. 3 illustrates several functions and capabilities of measuringmodule 210 as it relates to trending device 200 of the presentinvention. Measuring module 210 may be capable of performing suchfunctions as verifying a correct position of a lead within heart 212,passing current between electrodes of the lead at spaced time intervals214, measuring voltage between electrodes of the lead 216, calculatingimpedance values from the measured voltage 218, and storing impedancevalues 220.

[0036] Verifying the correct position of a lead within heart 212 mayinclude verifying that the lead is correctly positioned within a heartchamber, such as chambers 116-122 of FIG. 1 (leads 126 and 128), orwithin a vessel of the heart, such as vessel 124 shown in FIG. 1 (leads130 and 132). Verification of the correct position of the lead 212 maynot be a required function for the measuring module as the position ofthe lead may be assumed to be correct when an operator of device 200activates the device to begin measuring. In some cases, however,verification of that the lead is correctly positioned in the heart maybe part of the sensing capabilities of measuring module 210.

[0037] Passing current to electrodes of the lead at spaced timeintervals 214 may include passing current to one or more electrodes of alead within the heart, or to an electrode positioned within the heartand to a separate electrode position external the heart (step 215 inFIG. 3), such as, for example, electrodes 134, 136 shown in FIG. 1. Thecurrent may be passed to the electrodes of the lead at a constant rateor at spaced time intervals. The frequency in which current is passed toelectrodes of the lead may coincide with the voltage measurements beingtaken between the electrodes of the lead 216. The voltage measurementsmay also be taken between the lead electrode and the external electrode217. Preferably, current is provided to the electrodes so that voltagemeasurements can be taken at any desired time or time interval. Forexample, voltage measurements could be taken only during what wouldtypically be when the patient is sleeping, when the patient isexercising, or any number of combinations of time periods throughout agiven day, week, etc.

[0038] The measured voltage is then used for calculating impedancevalues 218. The calculated impedance values may be sent directly to theparameter module 230 shown in FIG. 4, stored within device 200, or maybe transferred to an outside source for storage. Storing impedancevalues 220 may include storing the impedance values into an array or alike format that reflects variables related to the voltage and impedancevalues.

[0039] The parameter module 230 may be capable of performing suchfunctions as collecting impedance values 232, averaging impedance valuesover set time intervals 234, calculating parameter values usingcalculated impedance values 236, storing calculated parameter values238, and transferring calculated parameter values 240 to, for example,an advanced patient management system 242 or to another outside source244.

[0040] Collecting impedance values may include accessing the storedimpedance values, for example, from a stored array of impedance values.The impedance values may be averaged over set time intervals prior tobeing used to calculate parameter values, or may be directly calculatedinto parameter values. Averaging impedance values over set timeintervals 234 may include averaging the impedance values on, forexample, a daily basis, a weekly basis, or other desired set timeinterval. The calculated parameter values may be stored within device200 for future processing by device 200, or for future transfer of theparameter values to an outside source. Calculated parameter values mayalso be directly transferred to a patient management system or to anoutside source that may, in other embodiments, perform the trending andanalyzing functions of modules 250 and 270.

[0041] The trending module 250, shown in FIG. 5, may be capable ofperforming several functions. For example, trending module 250 maycollect parameter values 252, trend collected parameter values over settime intervals 254, compare trends at different times 256, transfer datato a patient management system 258, transfer data to an outside source260, average parameter values over set time intervals 262, and trendaverage parameter values over set time intervals 264.

[0042] Collecting parameter values 252 may include collecting allparameter values stored by the parameter module 230, or collecting onlycertain parameter values at certain time intervals. Trending collectedparameter values over a set time interval 254 may coincide with whichparameter values are collected. Trending collected parameter values mayinclude determining changes in parameter values over certain timeintervals, such as, for example, changes in an average parameter valuefor each hour during a 24-hour period, for each day during a 7-day week,for each week during a given month, or for each month over the course ofa year, etc. A “trend” may be generally defined as a pattern over aperiod of time, such as, for example, a net increase over time, agradual, incremental increase over time, a steady value over time, etc.Comparing trends at different times 256 may not be required in allembodiments of trending module 250.

[0043] As stated above, averaging parameter values over set timeintervals 256 may be used for trending over set time intervals 264.Thus, either specific parameter values or average parameter values maybe compared to obtain trend data. Trend data may be transferred to anadvanced patient management system 258 or to another outside source 260that may be associated with device 200.

[0044] The trend data output by trending module 250 may be analyzed inseveral different ways. For example, trend data may be analyzed byanalyzing module 270 that is part of device 200. In other embodiments,an individual, or some type of analyzing system or module, such asexternal analyzing module 156 in FIG. 1, that is independent of device200, may perform analysis of trend data.

[0045] Analyzing module 270 may be capable of performing severalfunctions such as those shown in FIG. 6. For example, analyzing module270 may collect trend data 272, compare trend data 274, detectdifferences in trend data 276, and transfer trend data to a patientmanagement system 278. Analyzing module 270 may also track changes insympathetic activity 280, monitor effects of drug regimens 282, monitorprogress of congestive heart failure 284, detect occurrence ofmyocardial infarction 286, predict sudden cardiac death episode 288,store results 290, and transfer results to an outside source 292. Thefunctions of collecting trend data 272, comparing trend data 274 anddetecting differences in trend data 276 may involve further analysis andprocessing of trend information generated by trending module 250, theresults of which may be transferred, for example, to an advanced patientmanagement system 278 or another outside source 292. The trend data thatis collected, compared, and detected may be used to track certainphysiological indicators, such as indicators 280-288.

[0046] Trend data analyzed by analyzing module 270 may be generally usedto track or monitor sympathetic activity (tone) 280. Changes insympathetic activity, inferred from trend data may be useful diagnosticinformation for physicians. For example, the trend data may be used tomonitor the effects of a drug or neural stimulation regimen being givena patient to alter sympathetic activity. The trend data may also be usedto monitor the progress of congestive heart failure in a patient.Monitoring trend data related to intracardiac impedance could be usedinstead of R—R interval frequency spectrum (a conventional approach) orto augment such frequency-based sympathetic tone measurements.

[0047] Trend data may also be useful for detecting the occurrence ofmyocardial infarction 286. This type of detection is possible because amyocardial infarction typically triggers electrical remodeling whichleads to increased cardiac sympathetic nerve density. Thus, detectingthe occurrence of a myocardial infarction may be important becauseresearch has indicated that as many as one out of every three myocardialinfarctions are considered to be unnoticed by the patient. In addition,myocardial infarction is usually an eventual precursor to sudden cardiacdeath episode (SCD)

[0048] A further use of trend data may be in predicting SCD. Changes insympathetic activity, as may be inferred from certain types of trends insuch physiological parameters as described above, may indicate the onsetof an SCD. Early recognition by a patient or the patient's physician ofincreases of sympathetic activity over time (as indicated by trend data)may provide an opportunity for earlier treatment for the patient.

[0049] In some embodiments, the analyzing module of system 200 may beable to store results within device 200 for future transmission to anoutside source, or may immediately transfer results to an advancedpatient management system. Some advanced patient management systems mayinclude an alarm or similar indicator that would alert the patient orthe patient's physician if, for example, a certain threshold value ismet. Other patient management systems may be configured to connect to acommunications system, such as, for example, a telecommunicationssystem, the Internet via a hard landline or wireless network system, orsatellite system to automatically send patient data at spaced timeintervals or continuously send data in real time.

[0050] One example of a method of trending physiologic parameters isshown in FIG. 7. Method 300 may include the steps of measuringintracardiac impedance 310, deriving physiologic cardiac parameters 330,trending derived physiological parameters 350, and analyzing trend datato track predetermined physiological indicators 370. Each of steps 310,330, 350 and 370 may include steps or functions that coincide with thosefunctions described with reference to modules 210, 230, 250 and 270,respectively, and to systems 100 and 200 generally.

[0051] The functions performed by the system and method discussed abovemay be performed by a single unitary device, such as an implantablecardiac rhythm management device. The instructions for performing thesteps of the method and the functions related to the device discussedabove may be stored on a computer readable medium having computerexecutable instructions. The present invention may also include acomputer data signal embodied in a carrier wave readable by a computingsystem and encoding a computer program of instructions for executing acomputer program of instructions for executing a computer programperforming the method steps and system functions discussed above.

[0052] In some instances, various cardiac rhythm management (CRM)devices that are currently sold and marketed may be modified in order topractice the present invention. For example, if a given CRM deviceincludes hardware capable of performing necessary intracardiac impedancemeasurements, the software of the system may be modified or augmentedfor software that performs the impedance measuring, parameter deriving,trending and analyzing functions required by the present invention.

[0053] It is to be understood that the above description is intended tobe illustrative, and not restrictive. For example, the above-describedembodiments may be used in combination with each other. Many otherembodiments will be apparent to those of skill in the art upon reviewingthe above description. The scope of the invention should, therefore, bedetermined with reference to the appended claims, along with the fullscope of equivalents to which such claims are entitled. In the appendedclaims, the terms “including” and “in which” are used as theplain-English equivalents of the respective terms “comprising” and“wherein.”

We claim:
 1. A method of predicting sudden cardiac death comprising:determining intra-cardiac impedance; deriving a physiologic cardiacparameter from the determined impedance; trending the derivedphysiologic cardiac parameters over spaced time intervals; andpredicting the onset of a sudden cardiac death episode.
 2. The method ofclaim 1 wherein the trending step generates trended data and thepredicting step is based on the trended data.
 3. The method of claim 1wherein the predicting step comprises the steps of: comparing trends ofthe physiologic cardiac parameters; and detecting differences betweenthe trends.
 4. The method of claim 1 wherein the physiologic cardiacparameter is selected from a group comprising stroke volume, ejectionfraction and pre-ejection period.
 5. The method of claim 1 wherein thederiving step comprises the steps of: deriving a parameter when patientis at rest; and deriving a parameter when patient is not at rest.
 6. Themethod of claim 5 wherein the trending step comprises the step ofdetecting a difference between the parameters obtained at rest and theparameters obtained when the patient is not at rest.
 7. The method ofclaim 7 wherein the step of determining intra-cardiac impedancecomprises measuring intra-cardiac impedance with an implanted device byapplying a current between two electrodes of the device and measuring aresulting voltage that is used to calculate the intra-cardiac impedance.8. A system for predicting sudden cardiac death episode, comprising: ameasuring device that measures intra-cardiac impedance; a derivationmodule that derives a physiologic cardiac parameter from the measuredimpedance; and a trending module that trends the derived parameter overspaced time intervals to created trend data.
 9. The system of claim 8further comprising an analyzing module that analyzes the trend data topredict the onset of a sudden cardiac death episode.
 10. The system ofclaim 9 wherein analysis of the trends comprises comparing the trendsand detecting a difference between the trends.
 11. The system of claim 8further comprising a reporting module that reports the trends to anoutside source.
 12. The system of claim 11 wherein the reporting modulereports trends that predict the onset of a sudden cardiac death episode.13. The system of claim 8 wherein the derivation module and the trendingdevice are packaged with the implanted measuring device.
 14. The systemof claim 13 wherein the package is capable of being implanted in a humanbody.
 15. The system of claim 8 further comprising a device for storingthe trend data.
 16. The system of claim 8 wherein the physiologiccardiac parameter is selected from a group consisting of stroke volume,ejection fraction and pre-ejection period.
 17. The system of claim 8wherein the physiologic cardiac parameter correlates to sympathetic andparasympathetic activity.
 18. The system of claim 8 wherein the systemdownloads the trend data to a separate storage device.
 19. The system ofclaim 8 wherein the implanted device measures intra-cardiac impedance byapplying a current between two electrodes and measuring a resultingvoltage that is used to calculate the cardiac impedance.
 20. The systemof claim 19 wherein the electrodes are part of at least one unipolarlead and a remote device.
 21. The system of claim 19 wherein theelectrodes are part of at least one bipolar lead.
 22. The system ofclaim 19 wherein the electrodes are part of at least one unipolar leadand a bipolar lead.
 23. The system of claim 19 wherein the electrodesare part of at least one bipolar lead and a remote device.
 24. A methodof trending a cardiac parameter, comprising: measuring an intra-cardiacimpedance; deriving a physiologic cardiac parameter using the measuredimpedance; and trending the derived parameter over time.
 25. The methodof claim 24 wherein the measuring step comprises applying a current to alead positioned within the heart, determining a voltage as a result ofthe applied current, and calculating an impedance based on the voltage.26. The method of claim 24 wherein the impedance is measured at spacedtime intervals.
 27. The method of claim 24 wherein the physiologiccardiac parameter represents sympathetic nervous activity.
 28. Themethod of claim 24 wherein the trending step generates trend data, themethod further comprising the step of analyzing the trend data to trackpredetermined physiological indicators.
 29. The method of claim 28wherein tracking predetermined physiological indicators comprisespredicting a sudden cardiac death episode.
 30. The method of claim 28wherein tracking predetermined physiological indicators comprisesmonitoring a drug regimen.
 31. The method of claim 28 wherein trackingpredetermined physiological indicators comprises detecting theoccurrence of a myocardial infarction.
 32. The method of claim 28wherein tracking predetermined physiological indicators comprisesmonitoring progress of congestive heart failure.
 33. The method of claim24 wherein the deriving step comprises calculating the parameter usingthe measured impendence and storing the calculated impedance values intoan array.
 34. The method of claim 33 wherein the trending step comprisescomparing parameter values stored in the array.
 35. The method of claim28 further comprising the step of generating a signal when the trendingdata indicates that a threshold value for the predeterminedphysiological indicator has be met.
 36. The method of claim 28 furthercomprising the step of transmitting the trend data using acommunications system.
 37. The method of claim 28 further comprising thestep of transmitting the trend data to a patient management system. 38.The method of claim 24 wherein the measuring, deriving, and trendingsteps are completed by a unitary implanted device.
 39. Acomputer-readable medium having computer-executable instructions for themethod recited in claim
 24. 40. A computer data signal embodied in acarrier wave readable by a computing system and encoding a computerprogram of instructions for executing a computer program of instructionsfor executing a computer program performing the method recited in claim24.
 41. A device for trending a physiological cardiac parameter,comprising: an impedance module that measures an intra-cardiac impedanceat spaced time intervals; a parameter module that calculates cardiacparameter values using the measured impedance; a trending module thatgenerates trend data using the calculated parameter values.
 42. Thedevice of claim 41 wherein the parameter values represent a parameterselected from a group consisting of stroke volume, ejection fraction andpre-ejection period.
 43. The device of claim 41 wherein the trendingdata is used to predict a sudden cardiac death episode.
 44. The deviceof claim 41 further comprising an analyzing module that analyzes trenddata to track predetermined physiological indicators.
 45. The device ofclaim 44 wherein the predetermined physiological indicators comprisepredicting a sudden cardiac death episode.
 46. The device of claim 44wherein the predetermined physiological indicators comprise monitoringprogress of congestive heart failure.
 47. The device of claim 44 whereinthe predetermined physiological indicators comprise determining if amyocardial infarction has occurred.
 48. The device of claim 44 whereinthe predetermined physiological indicators comprise monitoring effectsof a drug regimen on the patient.
 49. The device of claim 44 wherein thepredetermined physiological indicators comprising monitoring changes insympathetic tone.